This invention concerns storage of flammable liquids such as gasoline, diesel, kerosene, lubricating oil, waste oil, solvents and other flammable liquids.
For many years, the usual practice has been to store these products in tanks located below ground level. This was done because underground tanks take less usable space, they are protected against collisions with motorized vehicles, they are less susceptible to vandalism, and they present less of a fire hazard. The disadvantage of underground storage is that if a leak occurs in the tank or its associated piping, it may be undetected for a long period of time, resulting in contamination of the surrounding soil. Leaking product may also diffuse through the soil to groundwater, resulting in the contamination of drinking water.
Because of this concern, federal, state, and local governments have passed regulations which require tank owners to install sensors for monitoring leaks, provide spill and overfill protection devices, improve tank and pipe construction, and demonstrate ability to pay for clean-up costs in the event of soil contamination. In some situations the regulations require the use of double-walled tanks and/or piping for underground storage. In this case, if the inner tank or piping fails, the product being stored is contained by the outer tank. Because the implementation of these requirements can be very costly, tank owners now have a strong incentive to store these flammable liquids in tanks which are located above ground.
The advantage of aboveground storage is that any leakage will be immediately apparent and corrective action can be taken before there is any substantial environmental degradation of the soil and ground water. Further, the piping associated with the tank may also be located above ground level so that a leak in the piping system would also be readily apparent. Because of these differences, aboveground tanks are not subject to the same strict requirements as are tanks located underground.
There are, however, a number of disadvantages with conventional aboveground storage as compared with underground storage. Because aboveground tanks are not surrounded by soil, there is more of a fire and explosion hazard. Aboveground tanks are subject to damage by collision with a motorized vehicle or other source, damage by vandals who might shoot at a tank, as well as being subject to the possibility of failure by developing a leak. If a tank were to fail due to any of these reasons, product would flow onto the ground, and depending on the size of the leak, it may flow away from the tank so as to cover a substantial area. If this liquid is ignited, a serious fire or explosion may result.
Because of this concern, conventional aboveground tanks storing flammable liquids are usually restricted from populated areas, and in addition, they are frequently surrounded with an external dike to contain the liquid within the dike. In some cases, conventional aboveground tanks are also fabricated with a double-wall construction in addition to, or as an alternative to, an external dike surrounding the tank.
If a conventional double walled tank or enclosing vault is used to reduce the hazards associated with conventional aboveground tanks, an explosion hazard can be created by the presence of heavier-than-air flammable vapors in the space between the inner tank and the enclosing vault or outer tank. Such vapors may result from spillage or leakage of fuel into that space. These vapors may form an explosive mixture creating considerable danger for a person checking the space for leakage.
If a conventional aboveground tank is exposed to a fire causing external heating of the tank, this heat is transferred to the flammable liquid contained in the tank. This heating will raise the vapor pressure inside the tank, and if the tank is not properly vented, the increased pressure can result in rupture of the tank. If the tank is vented to relieve this pressure, as these vapors leave the tank and mix with air, a combustible mixture is formed which may be ignited by the fire making it larger than it would otherwise have been. Further, wind may blow the burning vapors toward adjacent structures causing destruction of adjoining property.
Heating by direct flame impingement may also cause a reduction in the strength of the structural components of the tank, resulting in its rupture or the failure of its structural supports.
A further problem with conventional aboveground tanks is the diurnal heating of the tank during the day and cooling during the night. This heating and cooling results in what is known as tank "breathing" wherein the vapor pressure of the liquid in the tank increases during the day and decreases at night. As the temperature rises, the tank must be vented to prevent its being over pressurized. As a consequence, vapors in the tank are vented into the atmosphere resulting in atmospheric pollution. In a similar manner, when the tank cools during the night, air must be admitted into the tank to prevent its collapse. As this air is drawn into the tank, oxygen in the air mixes with the vapor already in the tank. Depending on the concentration of oxygen relative to the vapor concentration, an explosive mixture may be produced in the tank, which if ignited could result in the rupture of the tank through explosion. Oxygen and moisture drawn into the tank may also degrade the product contained in the tank.
Conventional vented aboveground tanks located in regions subject to high ambient temperature are capable of degrading the stored product, i.e., if the tank contents reach a temperature even as low as 80F., relatively volatile products such as gasoline are subject to evaporative degradation if exposed to these temperature conditions over a period of time, as little as a week. For a product containing a mixture of compounds, such as gasoline, the more volatile constituents evaporate leaving the less volatile compounds in the tank. After a period of time the product is no longer useful for its intended purpose.
Another disadvantage of conventional aboveground tanks in comparison with underground tanks in is the method of filling them. Underground tanks are commonly filled without their use of a pump, using gravity. Aboveground tanks are generally at the same level as the supply tank, so it is necessary to use a pump to transfer the product from the supply tank into the storage tank. Because of the possibility of overfilling the tank as a result of human error or equipment malfunction, it is highly desirable to provide contingency methods of preventing such overfill. Large tanks in particular are commonly filled at a high rate, and even a few seconds of inattention on the part of the operator could result in ten's if not hundred's of gallons of product flowing into the ground where it is both a fire and environmental hazard.
A tank system consisting of an inner tank surrounded by an enclosing concrete vault to protect the inner tank is currently in use. This tank system offers a degree of protection against collision and also provides some thermal insulation for the inner tank. However concrete has a tendency to form micro-cracks which prevent it from being acceptable as a material for constructing a leak-tight containment chamber. Because of this, these tanks do not offer true double-wall containment. Furthermore, a concrete vault cannot conveniently be tested for leaks. Also, the relatively great weight of a concrete vault greatly complicates its installation and substantially increases its installation cost. In addition, concrete decomposes in the intense heat of a fire, losing its structural strength. It is also subject to fracture if rapidly cooled such as by water from, a fire hose used in the process of extinguishing a fire.
In some regions of the country, there is currently a requirement for vapor recovery systems in which displaced vapor from the fuel tank of a vehicle receiving fuel dispensed from the tank is directed into the storage tank via an annular space in a coaxial fueling hose. Any liquid fuel inadvertently splashing into this annular space must be removed or the effectiveness of the vapor recovery system is greatly diminished. If the supply tank is located underground, this is accomplished by a liquid/vapor separation device-located at the bottom of the dispensing pump. The separated liquid flows by gravity back into the storage tank. Unless the liquid/vapor separation device is located on the top of the tank, this method does not work for a conventional aboveground tank because the level of liquid in the tank is higher than the liquid/vapor separation device. It is not convenient to locate the liquid/vapor device at the top of the tank because the dispensing pump will be so far above ground level that it cannot conveniently be reached. In addition, in order for the trapped liquid to reach the separation device, it is necessary that the return vapor force the trapped liquid, against gravity, from the level of the fill nozzle to the level of the liquid/vapor separation device at the top of the tank. This would cause increased back-pressure in the vapor return line, greatly diminishing the effectiveness of the vapor recovery system.